Second Order Active Filters

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2nd order active filtering has two main advantages:

  • High impedance input, low impedance output
  • greater attenuation at high range (-40dB/decade as opposed to -20dB/decade for RC filter)

The high input impedance provided by an op amp circuit protects the instrument providing the signal from passing excessive current which could cause a voltage drop in the measurement signal or damage to the instrument itself.

Because the filter is second order, the rolloff after the cutoff frequency on a bode plot of a 2nd order low pass filter for example, is -40 dB/decade as opposed to -20 dB/decade for a first order (RC) filter which means significantly greater attenuation outside the pass band and a sharper cutoff. This can be very useful if the noise frequency is close to the desired signal frequency.

2ndOrderFilterDiagrams.gif

(http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm)

Where the cutoff frequency for both high and low pass filters in Hz is:

f = 1 / (2 * pi * sqrt(R1R2C1C2))



External Links

http://en.wikipedia.org/wiki/Sallen_Key_filter

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